Merits and limitations of recombinant models for the study of human P450-mediated drug metabolism and toxicity: an intralaboratory comparison

Efficient substrate screening and inhibitor testing of human CYP4Z1 using permeabilized recombinant fission yeast

We have established a protocol for the preparation of permeabilized fission yeast cells (enzyme bags) that recombinantly express human cytochrome P450 enzymes (CYPs). A direct comparison of CYP3A4 activity gave an eightfold higher space-time yield for enzyme bag-catalyzed biotransformation as compared to whole-cell biotransformation, even though the total number of cells employed was lower by a factor of 150. Biotransformation of the luminogenic substrate Luciferin-H using CYP2C9-containing enzyme bags proceeded efficiently and stably for 24h. CYP4Z1 is of interest because it is strongly overexpressed both in breast cancer cells and in breast cancer metastases; however, current knowledge about its catalytic properties is very limited. Screening of CYP4Z1-containing enzyme bags with 15 luminogenic substrates enabled us to identify two new hydroxylations and eleven ether cleavage reactions that are catalyzed by CYP4Z1. By far the best substrate found in this study was Luciferin benzyl ether (Luciferin-BE). On the basis of the recently published crystal structure of CYP4B1 we created a new homology model of CYP4Z1 and performed molecular docking experiments, which indicate that all active substrates show a highly similar binding geometry compared to the endogenous substrates. The model predicts that Ser113, Ser222, Asn381, and Ser383 are key hydrogen bonding residues. We also identified five new inhibitors of CYP4Z1: miconazole, econazole, aminobenzotriazole, tolazoline, and 1-benzylimidazole respectively, with the last compound being the most potent giving an IC₅₀ value of 180nM in our test system.

Improved expression of recombinant cytochrome P450 monooxygenase in Escherichia coli for asymmetric oxidation of sulfides

Escherichia coli BL21 as production strain for the production of cytochrome P450 monooxygenase (P450SMO) from Rhodococcus sp. in high yields was developed. The expression was first optimized with a series of flask experiments testing several key parameters for their influence on the expression level and enzyme activity. The optimal process parameters found in the flask experiments were verified in a cultivation process in a 5-L bioreactor. Glycerol proved to be superior over glucose as carbon source. Low dissolved oxygen (DO) concentration (<10%) during expression was found to be critical for active P450s production, resulting in expression level of 400 nM for P450SMO. Intact cells were used to establish an efficient bioconversion system for the production of sulfoxidation product. With p-chlorothioanisole as a representative substrate, the desired product (S-sulfoxide) was afforded with 99% ee and highest production of 130 mg/L within 12 h.

Effects of N-terminal modification of recombinant human cytochrome P450 1A2 on catalytic activity

1. The high-level expression of mammalian cytochrome P450 in bacteria usually requires modification of the amino-terminal region of the enzyme. The effect of altering amino acids in the N-terminus of human recombinant CYP1A2 on its catalytic activity was investigated herein. 2. Rates of 7-ethoxyresorufin O-deethylation by CYP1A2a (a form made by altering the amino acids LLL of CYP1A2 to RER at positions 3-5) in reconstituted systems were significantly low compared with those of other CYP1A2 N-terminal variants at a low ratio of cytochrome P450 to NADPH-cytochrome P450 reductase, but not at higher reductase concentrations. 3. CYP1A2a-dependent ethoxyresorufin O-deethylase activity in a cumene hydroperoxide-supported system was approximately 2-fold higher than other CYP1A2 N-terminal variants. 4. Our results suggest that modification of three N-terminal amino acids in CYP1A2 alters the interaction between CYP1A2 and the reductase in reconstituted phospholipid vesicles and in the bicistronic membranes.

A Skin-Like Cytochrome P450 Cocktail Activates Prohaptens to Contact Allergenic Metabolites

Allergic contact dermatitis is a complex syndrome representing immunological responses to cutaneous exposure to protein-reactive chemicals. Although many contact sensitizers directly can elicit this disorder, others (prohaptens) require activation. Knowledge regarding the activating mechanisms remains limited, but one possibility is metabolic activation by cytochrome P450 (CYP) enzymes in the skin. We have, after quantitative reverse transcriptase-PCR studies of the CYP content in 18 human skin samples, developed an enriched skin-like recombinant human (rh) CYP cocktail using CYP1A1, 1B1, 2B6, 2E1, and 3A5. To validate the rhCYP cocktail, a prohaptenic conjugated diene ((5R)-5-isopropenyl-2-methyl-1-methylene-2-cyclohexene) was investigated using: the skin-like rhCYP cocktail, a liver-like rhCYP cocktail, single rhCYP enzymes, liver microsomes, keratinocytes, and a dendritic cell (DC) assay. The diene was activated to sensitizing epoxides in all non-cell-based incubations including the skin-like rhCYP cocktail. An exocyclic epoxide metabolite ((7R)-7-isopropenyl-4-methyl-1-oxaspiro[2.5]oct-4-ene) was found to be mainly responsible for the allergenic activity of the diene. This epoxide also induced pronounced DC activation indicated by upregulation of IL-8. The skin-like rhCYP cocktail provides a simplified alternative to using skin tissue preparations in mechanistic studies of CYP-mediated skin metabolism of prohaptens and offers the future possibility of designing in vitro predictive assays for assessment of allergenic activity of prohaptens.

The Evolving Role of Drug Metabolism in Drug Discovery and Development

Drug metabolism in pharmaceutical research has traditionally focused on the well-defined aspects of absorption, distribution, metabolism and excretion, commonly-referred to ADME properties of a compound, particularly in the areas of metabolite identification, identification of drug metabolizing enzymes (DMEs) and associated metabolic pathways, and reaction mechanisms. This traditional emphasis was in part due to the limited scope of understanding and the unavailability of in vitro and in vivo tools with which to evaluate more complex properties and processes. However, advances over the past decade in separate but related fields such as pharmacogenetics, pharmacogenomics and drug transporters, have dramatically shifted the drug metabolism paradigm. For example, knowledge of the genetics and genomics of DMEs allows us to better understand and predict enzyme regulation and its effects on exogenous (pharmacokinetics) and endogenous pathways as well as biochemical processes (pharmacology). Advances in the transporter area have provided unprecedented insights into the role of transporter proteins in absorption, distribution, metabolism and excretion of drugs and their consequences with respect to clinical drug-drug and drug-endogenous substance interactions, toxicity and interindividual variability in pharmacokinetics. It is therefore essential that individuals involved in modern pharmaceutical research embrace a fully integrated approach and understanding of drug metabolism as is currently practiced. The intent of this review is to reexamine drug metabolism with respect to the traditional as well as current practices, with particular emphasis on the critical aspects of integrating chemistry and biology in the interpretation and application of metabolism data in pharmaceutical research.

Role of Hepatic Cytochrome P450s in the Pharmacokinetics and Toxicity of Cyclophosphamide: Studies with the Hepatic Cytochrome P450 Reductase Null Mouse

Cyclophosphamide (CPA) is an anticancer prodrug that is dependent on cytochrome P450 (CYP) metabolism for its therapeutic effectiveness. In spite of the use of CPA in the clinic for over 50 years, little is known about the relationship between its toxicokinetics and therapeutic response. We have employed a powerful new model, the Hepatic Cytochrome P450 Reductase Null (HRN) mouse, which has almost no hepatic cytochrome P450 activity, to study the toxicokinetics of CPA and to establish in vivo the role of hepatic P450 metabolism in its pharmacokinetics. In HRN mice the in vitro metabolism and intrinsic clearance of CPA was over 6-fold lower than in wild-type animals. This change in CPA metabolism was also reflected in vivo, with a profound difference in the pharmacokinetics of both CPA and its metabolites. At a CPA dose of 100 mg/kg, the Cmax, plasma area under the curve (AUC) and half-life were increased by 2.6-, 6.2-, and 3.2-fold, respectively, in the HRN mice. Similar changes were also observed at a dose of 300 mg/kg. These data confirm that hepatic metabolism is the major route of CPA elimination and disposition. The primary metabolites of CPA, 4-hydroxycyclophosphamide (4-OH-CPA) and 3-dechloroethylcyclophosphamide, were still formed, but at altered rates in the HRN mice. At 100 mg/kg the t1/2 for 4-OH-CPA was increased 1.8-fold, the Cmax reduced 1.7-fold, and the AUC remained unchanged. This latter finding shows that P450-mediated oxidative metabolism is essential for the clearance of this compound. Toxicokinetic analysis of CPA-induced myelosuppression and granulocytopenia showed that at high doses (> or =100 mg/kg) there was no difference in myelotoxicity between the wild-type and HRN mice. However, at lower doses (< or =70 mg/kg) a significant difference was observed, with little toxicity seen in HRN mice but at least a 45% reduction in the bone marrow granulocyte population in wild-type mice. Meta-analysis of the toxicity experiments showed the myelotoxicity of CPA was found to be closely correlated with the Cmax of 4-OH-CPA (r2= 0.80, P = 0.002). As the therapeutic effectiveness of CPA has been linked to the AUC for 4-OH-CPA, the finding that 4-OH-CPA Cmax may determine its level of myelotoxicity indicates that the therapeutic index could be altered by changing the method of CPA administration. Furthermore, monitoring 4-OH-CPA Cmax may identify individuals at most risk of CPA side effects.

Preparative synthesis of drug metabolites using human cytochrome P450s 3A4, 2C9 and 1A2 with NADPH-P450 reductase expressed in Escherichia coli

Three human cytochrome P450s, 3A4, 2C9 and 1A2, were each co-expressed with NADPH-P450 reductase in Escherichia coli and used in the preparative synthesis of drug metabolites. Low dissolved oxygen (DO) concentration (<1%) during expression was found to be critical for producing active P450s. Control of temperature, pH and glycerol supplementation in 10-L fermentations enhanced enzyme expression 31-86%. Additional improvements were obtained by altering media formulations, resulting in bicistronic expression levels of 890, 1,800 and 1,010 nmol/L for 3A4, 2C9 and 1A2, respectively. The P450 titers achieved in fermentors exceeded those in flask fermentations by 3- to 6-fold in this study and up to 10-fold when compared with previously reported literature. Intact cells and isolated membranes obtained from 10-L fermentations were used to establish an efficient bioconversion system for the generation of metabolites. To demonstrate the utility of this approach, known metabolites of the anabolic steroid testosterone, the anti-inflammatory agent diclofenac and the analgesic agent phenacetin, were generated using 3A4, 2C9 and 1A2, respectively. The reaction conditions were optimized for pH, temperature, DO concentration, use of co-solvent and glucose supplementation. Conversion yields of 29-93% were obtained from 1-L reactions, enabling isolation of 59 mg 6beta-hydroxytestosterone, 110 mg 4'-hydroxydiclofenac and 88 mg acetaminophen.

A STRATEGY FOR INVESTIGATING THE CYP SUPERFAMILY USING TARGETED ANTIBODIES IS A PARADIGM FOR FUNCTIONAL GENOMIC STUDIES

In this review we describe the use of targeted antibodies developed to facilitate studies on the expression of P450 proteins. The challenge of producing specific antibodies that distinguish between often highly related P450 proteins has led to the development of methods of antibody production to meet this need. Targeting antibodies toward the C terminus of P450 proteins has been found to be a particularly successful approach that is both rapid and efficient at producing specifically binding antibodies. Recent advances in genomic sequencing and proteomics now allow ready identification of expressed proteins. The levels and distributions of these proteins may be determined using antibody-based methods. However, for each protein to be studied, a unique antibody will be required. Consequently, some means of producing large numbers of well defined antibodies is needed. In this context, the potential of extending the approach used to produce specific antibodies against P450 proteins to the wider field of functional genomics is discussed.

The N-demethylation of the doxepin isomers is mainly catalyzed by the polymorphic CYP2C19

PURPOSE

This study was conducted to identify the cytochrome P450s (CYPs) responsible for the metabolism of the cis- and trans-isomers of the tricyclic antidepressant doxepin to its pharmacologically active N-desmethylmetabolite by in vitro techniques.

METHODS

The doxepin N-demethylation was studied by means of pooled human liver microsomes and chemical inhibitors, recombinant human (rh)-CYPs, and geno- and phenotyped human liver microsomes.

RESULTS

The N-demethylation of both isomers was inhibited most prominently by tranylcypromine (CYP2C19) to more than 50%. Furafylline (CYP1A2) and sulfaphenazole (CYP2C9) inhibited the N-demethylation to a lesser extent while quinidine (CYP2D6) or troleandomycine (CYP3A4) had no effect. Rh-CYP2C19, -CYP1A2, and -CYP2C9 were able to N-demethylate cis- and trans-doxepin. Only traces of trans-desmethyldoxepin were detectable when CYP3A4 was used. The maximum velocity in the cis- and transdoxepin N-demethylation was significantly (P < 0.05) lower in microsomes with low CYP2C19 activity (345 +/- 44 and 508 +/- 75 pmol/min/ mg protein, respectively) compared to those with high CYP2C19 activity (779 +/- 132 and 1,189 +/- 134 pmollmin/mg).

CONCLUSION

The present study demonstrates a significant contribution of the polymorphic CYP2C19 to the N-demethylation of doxepin. CYP2C9 and CYP1A2 play a minor role and CYP3A4 does not contribute substantially.

In vitro investigation of cytochrome P450-mediated metabolism of dietary flavonoids

Human and mouse liver microsomes and membranes isolated from Escherichia coli, which expressed cytochrome P450 (CYP) 1A2, 3A4, 2C9 or 2D6, were used to investigate CYP-mediated metabolism of five selected dietary flavonoids. In human and mouse liver microsomes kaempferol, apigenin and naringenin were hydroxylated at the 3'-position to yield their corresponding analogs quercetin, luteolin and eriodictyol, whereas hesperetin and tamarixetin were demethylated at the 4'-position to yield eriodictyol and quercetin, respectively. Microsomal flavonoid metabolism was potently inhibited by the CYP1A2 inhibitors, fluvoxamine and -naphthoflavone. Recombinant CYP1A2 was capable of metabolizing all five investigated flavonoids. CYP3A4 recombinant protein did not catalyze hesperetin demethylation, but showed similar metabolic profiles for the remaining compounds, as did human microsomes and recombinant CYP1A2, although the reaction rates in general were lower as compared to CYP1A2. CYP2C9 catalyzed the 4'-demethylation of tamarixetin, whereas CYP2D6 did not seem to play any role in the metabolism of the selected flavonoids. The major involvement in flavonoid metabolism of human CYP1A2, which mediates the formation of metabolites with different biochemical properties as compared to the parent compound and furthermore is known to be expressed very differently among individuals, raises the important question of whether individual differences in the CYP enzyme activity might affect the beneficial outcome of dietary flavonoids, rendering some individuals more or less refractory to the health-promoting potential of dietary flavonoids.

Establishment of the transformants expressing human cytochrome P450 subtypes in HepG2, and their applications on drug metabolism and toxicology

Transformants with stable expression of a series of human cytochrome P450 (CYP) subtypes in the human hepatic cell line, HepG2, were established. These transformants are designated Hepc/1A1.4, Hepc/1A2.9, Hepc/2A6L.14, Hepc/2B6.68, Hepc/2C8.46, Hepc/2C9.1, Hepc/2C19.12, Hepc/2D6.39, Hepc/2E1.3-8 and Hepc/3A4.2-30, which stably expressed human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4, respectively. The expression of the CYP subtypes in the transformants was confirmed by both determination of enzyme activities and the reverse transcriptase polymerase chain reaction (RT-PCR) procedure. The apparent K(m) values of the expressed CYP subtypes for their specific substrates were close to those of human liver microsomes. In addition to their CYP activities, these transformants retained glucuronide- and sulfate-conjugating activities. Furthermore, the activities of CYP2C9, CYP2D6 and CYP3A4 were inhibited by their specific inhibitors. The cytotoxicity of acetaminophen (APAP), cyclophosphamide (CPA) and benz[a]anthracene (BA) were analyzed by CYP-expressing transformants. The cytotoxicity depended on the expression of CYP subtypes and increased in a dose-dependent manner. These results show the metabolic activation of APAP, CPA and BA by the specific CYP subtypes expressed in the transformants and demonstrate the usefulness of these transformants for in vitro metabolic and toxicological studies in human liver.

Contributions of five human cytochrome P450 isoforms to the N-demethylation of clozapine in vitro at low and high concentrations

The authors assessed the in vitro contribution of cytochrome P450 (CYP) isoforms 1A2, 3A4, 2C9, 2C19, and 2D6 to the N-demethylation of clozapine mediated by human liver microsomal preparations (HLM). In contrast to previous studies, the authors focused on a relatively low hepatic concentration level, 5 microM, to assess the conditions at a therapeutically relevant hepatic concentration level of clozapine. The optimal concentrations of specific inhibitors were initially established using cDNA-expressed CYP isoforms. The mean contributions of CYPs 1A2, 2C19, 3A4, 2C9, and 2D6 amounted to 30%, 24%, 22%, 12%, and 6%, respectively, with regard to the total HLM-mediated N-demethylation. Thus, the present in vitro study on clozapine N-demethylation suggests that CYP1A2 is the most important form at low concentrations, which is in agreement with clinical findings. CYP2C19 is also of considerable importance, while the roles of CYP2C9 and 2D6 are more modest. CYP3A4 attained a dominating role with an average contribution of 37% at a high clozapine concentration (50 microM). The rate of other metabolic routes mediated by CYP2D6 only corresponded to about one fifth of the CYP2D6 catalyzed N-demethylation rate.

Differential effects of fluvoxamine and other antidepressants on the biotransformation of melatonin

Melatonin, the predominant product of the pineal gland, is involved in the maintenance of diurnal rhythms. Nocturnal blood concentrations of melatonin have been shown to be enhanced by fluvoxamine, but not by other serotonin reuptake inhibitors. Because fluvoxamine is an inhibitor of several cytochrome P450 (CYP) enzymes, the authors studied the biotransformation of melatonin and the effects of fluvoxamine on the metabolism of melatonin in vitro using human liver microsomes and recombinant human CYP isoenzymes. Melatonin was found to be almost exclusively metabolized by CYP1A2 to 6-hydroxymelatonin and N-acetylserotonin with a minimal contribution of CYP2C19. Both reactions were potently inhibited by fluvoxamine, with a Ki of 0.02 microM for the formation of 6-hydroxymelatonin and 0.05 microM for the formation of N-acetylserotonin. Other than fluvoxamine, fluoxetine, paroxetine, citalopram, imipramine, and desipramine were also tested at 2 and 20 microM. Among the other antidepressants, only paroxetine was able to affect the metabolism of melatonin at supratherapeutic concentrations of 20 microM, which did not reach by far the magnitude of the inhibitory potency of fluvoxamine. The authors concluded that fluvoxamine is a potent inhibitor of melatonin degradation. Because this inhibitory action is also found in vivo, fluvoxamine might be used as an enhancer of melatonin, which might offer new therapeutic possibilities of fluvoxamine.

Engineering of a functional human NADH-dependent cytochrome P450 system

A functional human NADH-dependent cytochrome P450 system has been developed by altering the cofactor preference of human NADPH cytochrome P450 reductase (CPR), the redox partner for P450s. This has been achieved by a single amino acid change of the conserved aromatic amino acid Trp-676, which covers the re-side of the FAD isoalloxazine ring in the nicotinamide-binding site. Of the mutations made, the substitution of Trp-676 with alanine (W676A) resulted in a functional NADH-dependent enzyme, which catalyzed the reduction of cytochrome c and ferricyanide as well as facilitated the metabolism of 7-ethoxyresorufin by CYP1A2. Kinetic analysis measuring cytochrome c activity revealed that the NADH-dependent k(cat) of W676A is equivalent (90%) to the NADPH-dependent k(cat) of the wild-type enzyme, with W676A having an approximately 1,000-fold higher specificity for NADH. The apparent K(M)(NADPH) and K(M)(NADH) values of W676A are 80- and 150-fold decreased, respectively. In accordance with structural data, which show a bipartite binding mode of NADPH, substitution of Trp-676 does not affect 2'-AMP binding as seen by the inhibition of both wild-type CPR and the W676A mutant. Furthermore, NADPH was a potent inhibitor of the W676A NADH-dependent cytochrome c reduction and CYP1A2 activity. Overall, the results show that Trp-676 of human CPR plays a major role in cofactor discrimination, and substitution of this conserved aromatic residue with alanine results in an efficient NADH-dependent cytochrome P450 system.

Present and future in vitro approaches for drug metabolism

The 1980s through 1990s witnessed the widespread incorporation of in vitro absorption, distribution, metabolism, and excretion (ADME) approaches into drug development by drug companies. This has been exemplified by the integration of the basic science of cytochrome P450s (CYPs) into most drug metabolism departments so that information on the metabolic pathways of drugs and drug-drug interactions (DDIs) is no longer an academic exercise, but essential for regulatory submission. This has come about due to the application of a variety of new technologies and in vitro models. For example, subcellular fractions have been widely used in metabolism studies since the 1960s. The last two decades has seen the increased use of hepatocytes as the reproducibility of cell isolations improved. The 1990s saw the rejuvenation of liver slices (as new slicers were developed) and the utilization of cDNA expressed enzymes as these technologies matured. In addition, there has been considerable interest in extrapolating in vitro data to in vivo for parameters such as absorption, clearance and DDIs. The current philosophy of drug development is moving to a 'fail early--fail cheaply' paradigm. Therefore, in vitro ADME approaches are being applied to drug candidates earlier in development since they are essential for identifying compounds likely to present ADME challenges in the latter stages of drug development. These in vitro tools are also being used earlier in lead optimization biology, in parallel with approaches for optimizing target structure activity relationships, as well as identification of DDI and the involvement of metabolic pathways that demonstrate genetic polymorphisms. This would suggest that the line between discovery and development drug metabolism has blurred. In vitro approaches to ADME are increasingly being linked with high-throughput automation and analysis. Further, if we think of perhaps the fastest available way to screen for successful drugs with optimal ADME characteristics, then we arrive at predictive computational algorithms, which are only now being generated and validated in parallel with in vitro and in vivo methods. In addition, as we increase the number of ADME parameters determined early, the overall amount of data generated for both discovery and development will increase. This will present challenges for the efficient and fast interpretation of such data, as well as incorporation and communication to chemistry, biology, and clinical colleagues. This review will focus on and assess the nature of present in vitro metabolism approaches and indicate how they are likely to develop in the future.

Polymorphisms in P450 CYP1B1 affect the conversion of estradiol to the potentially carcinogenic metabolite 4-hydroxyestradiol

Most drug metabolizing cytochrome P450s (P450) are predominantly expressed in the liver. In contrast, human CYP1B1 is an extrahepatic P450 which is overexpressed in many tumours and has been strongly implicated in the activation of carcinogens. Rare allelic variants of the CYP1B1 gene which encode an inactive protein have been identified. However, four polymorphisms which most likely do not abolish functionality have been described. In this report, we have characterized the functional consequences of these. A CYP1B1 cDNA, identical to a cDNA published previously, served as a template to introduce allelic changes either separately or in combination. The resulting effects on CYP1B1 activity were determined in membranes isolated from Escherichia coli which coexpressed CYP1B1 together with P450 reductase. None of the allelic changes affected the CYP1B1 expression level. The allelic changes Arg48 to Gly, Ala19 to Ser and Asn453 to Ser had little influence on the Vmax and the Km of the CYP1B1 mediated 2- and 4-hydroxylation of estradiol. In contrast, the Km of these metabolic pathways was increased at least three-fold by the allelic change Va432 to Leu or by simultaneously changing Val432 to Leu and Asn453 to Ser. However, these alterations had little effect on the kinetic parameters of other CYP1B1 mediated reactions such as the epoxidation of (-)-trans-(7R,8R)-benzo[a]pyrene 7,8-dihydrodiol as determined by (r-7,t-8,t-9,c-10)-benzo[a]pyrene tetraol formation, or such as the O-dealkylation of ethoxyresorufin and the 1'-hydroxylation of bufuralol. Molecular modelling suggests that amino acid residue 432 of CYP1B1 may be involved in the interaction between CYP1B1 and P450 reductase. Since 4-hydroxyestradiol has been implicated in hormonal carcinogenesis and CYP1B1 is expressed in target tissues, the data presented demonstrate that polymorphisms in CYP1B1 have the potential to affect disease susceptibility.

Recombinant in vitro tools to predict drug metabolism and safety

Drug metabolism determines several pharmacological and toxicological properties of pharmaceuticals and is catalysed by drug metabolizing enzymes. Prediction of drug metabolism in humans based on animal experiments is complicated by species differences in the catalytic properties of these enzymes. This review describes and evaluates the use of recombinant models that contain human drug metabolizing enzymes to facilitate the prediction of pharmacokinetic properties of candidate drugs in humans.